1. Field of the Invention
The present invention relates to an image forming apparatus and method.
2. Description of the Related Art
Many conventional printers, copiers, facsimile machines, and so on employ an electrophotographic printing apparatus having a photosensitive drum. The surface of the drum is uniformly charged, then exposed to light in accordance with printing data to form a latent electrostatic image. The latent image is developed by application of toner particles, which are then transferred to paper and fused by heat and pressure to form a printed image.
A problem with this type of image forming apparatus is that printed images with, for example, large solid black areas consume excessive amounts of toner, making the printing process expensive. A similar problem occurs in printing apparatus that ejects drops of ink onto paper: images with large printed areas are expensive to print because they consume much ink. Natural or photographic color images, in which the entire surface of the paper may be covered with various colors of toner or ink, are particularly expensive to print.
The cost of printing such images can be reduced by decimating the printing data, that is, by selectively masking some of the bits in the final bit-mapped form of the printing data. If the selective masking process is carried out by software, however, considerable processing time is required and the printing speed is slowed accordingly. Furthermore, decimation may lead to discontinuities and other unwanted results. Japanese Unexamined Patent Application Publication No. 2000-301768 discloses a hardware method of decimation that avoids the delays of software decimation and also avoids unwanted discontinuities in the printed output.
To complicate the problem, the image may be formed on overhead projector (OHP) film instead of paper. To sharpen the image projected by the overhead projector, application programs that generate OHP images may process the image data so as to increase the density of, for example, background areas and reduce the density of, for example, text displayed on the background. As a result, different areas require different degrees of decimation, but it is difficult for either hardware or software to distinguish between the different types of areas. If the decimation process is omitted for OHP images, however, dense background areas can make these images very expensive to print.
More generally, whenever an image is processed in a particular way to improve the quality of the image, the improvement tends to interfere with subsequent decimation processing carried out to reduce the cost of printing the image. If an attempt is made to compensate for the effects of image processing, the decimation process becomes complex and inefficient.